// Linux 线程池

#pragma once
#ifndef THREAD_POOL_H
#define THREAD_POOL_H

#include <iostream>
#include <stdlib.h>
#include <queue>
#include <mutex>			  // 锁
#include <thread>			  // 线程
#include <condition_variable> // 条件变量
#include <functional>		  // 接受所有类似函数的东西
#include <future>			  // 来自未来的值
#include <atomic>			  // 原子操作
#include <memory>			  // 智能指针
#include <stdexcept> 	      // C++异常

static const size_t THREAD_SIZE = 5; // 默认线程数量

// 线程池
class ThreadPool
{
public:
	ThreadPool(size_t);
	template <class F, class... Args>
	auto enqueue(F &&f, Args &&...args)
		-> std::future<typename std::result_of<F(Args...)>::type>;
	~ThreadPool();

private:
	// need to keep track of threads so we can join them
	std::vector<std::thread> workers;
	// the task queue
	std::queue<std::function<void()>> tasks;

	// synchronization
	std::mutex queue_mutex;
	std::condition_variable condition;
	std::atomic<bool> stop;
};

// the constructor just launches some amount of workers
inline ThreadPool::ThreadPool(size_t threads = THREAD_SIZE)
	: stop(false)
{
	threads = threads < 1 ? 1 : threads;
	for (size_t i = 0; i < threads; ++i)
		workers.emplace_back(
			[this]
			{
				for (;;)
				{
					std::function<void()> task;

					{
						std::unique_lock<std::mutex> lock(this->queue_mutex);
						this->condition.wait(lock,
											 [this]
											 { return this->stop || !this->tasks.empty(); });
						if (this->stop && this->tasks.empty())
							return;
						task = std::move(this->tasks.front());
						this->tasks.pop();
					}

					task();
				}
			});
}

// add new work item to the pool
template <class F, class... Args>
auto ThreadPool::enqueue(F &&f, Args &&...args)
	-> std::future<typename std::result_of<F(Args...)>::type>
{
	using return_type = typename std::result_of<F(Args...)>::type;

	auto task = std::make_shared<std::packaged_task<return_type()>>(
		std::bind(std::forward<F>(f), std::forward<Args>(args)...));

	std::future<return_type> res = task->get_future();
	{
		std::unique_lock<std::mutex> lock(queue_mutex);

		// don't allow enqueueing after stopping the pool
		if (stop)
			throw std::runtime_error("enqueue on stopped ThreadPool");

		tasks.emplace([task]()
					  { (*task)(); });
	}
	condition.notify_one();
	return res;
}

// the destructor joins all threads
inline ThreadPool::~ThreadPool()
{
	this->stop.store(true);

	condition.notify_all();
	for (std::thread &worker : workers)
		worker.join();
}

#endif

